Adaptive current limit for power factor correction

ABSTRACT

In one embodiment, an apparatus for performing power factor correction is provided. A power factor corrector includes an input configured to sense a current from an input circuit. A reference generator generates a current limit based on an input voltage. The current limit reference is dynamically changed based on the input voltage. A control signal generator controls the current in the input circuit based on a comparison of the current and the generated current limit.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional App. No.61/169,922 for “Constant Power Limit for Power Factor Correction” filedApr. 16, 2009, the contents of which is incorporated herein by referencein their entirety.

BACKGROUND

The present disclosure generally relates to power factor correction.

Unless otherwise indicated herein, the approaches described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Power factor is a ratio of real power flowing to a load to apparentpower. Power factor may be described as a number between 0 and 1 orexpressed as a percentage. It is desirable to have the power factor becloser to 1 or 100%.

Two factors may affect the power factor. A displacement factor is when acurrent waveform is not in-phase with a voltage waveform. A distortionfactor is when the current waveform is not sinusoidal; that is,distortion may be present in the current waveform. Power factorcorrection may be used to correct these two factors.

FIG. 1 shows two waveforms illustrating effects of the displacement anddistortion factors. A voltage waveform 102 and two current waveforms 104a and 104 b are shown. The two current waveforms 104 a and 104 billustrate the displacement factor and the distortion factor separately.

Current waveform 104 a shows the displacement factor. A phase difference1 exists between voltage waveform 102 and current waveform 104 a.Current waveform 104 a is thus delayed with respect to voltage waveform102.

Current waveform 104 b shows the distortion factor. Current waveform 104b is in phase with voltage waveform 102; however, current waveform 104 bis distorted. For example, total harmonic distortion (THD) is present.

A combination of the displacement factor and distortion factor causesthe power factor to be lower. Power factor correction is used to shapethe current waveform to make it sinusoidal and in-phase with voltagewaveform 102, which raises the power factor.

During power factor correction, it may be desirable to limit the maximumcurrent in an input circuit. If the current is not limited, a system maybe damaged. For a given system, input power is given by the root meansquare (rms) of the input voltage V_(in rms) multiplied by the root meansquare of the input current I_(in rms). The range of the input voltageV_(in rms) is typically 85V-277V. The maximum current occurs at theminimum input voltage V_(in rms) of the range for constant input power.FIG. 2 shows a graph 200 of input power vs. input voltage V_(in rms) forconstant input current. As shown, as the input voltage V_(in rms)increases, the input power increases. At a point 202, the input voltageV_(in rms) is at its lowest and the current is expected to be at itshighest. The maximum current limit is thus set at a percentage above thecurrent that occurs at point 202 because this is expected to be themaximum current over the range of voltages for the input voltageV_(in rms). The maximum current limit is constant and does not change.Because the limit is constant, this may lead to high input power overthe voltage range when the current limit is reached at every switchingcycle of a switched mode power supply.

SUMMARY

In one embodiment, an apparatus for performing power factor correctionis provided. A power factor corrector includes an input configured tosense a current from an input circuit. A reference generator generates acurrent limit based on an input voltage. The current limit reference isdynamically changed based on the input voltage. A control signalgenerator controls the current in the input circuit based on acomparison of the current and the generated current limit.

In one embodiment, an apparatus is provided comprising: an inputconfigured to sense a current and input voltage from an input circuit; areference generator configured to generate a current limit based on theinput voltage, wherein the current limit is adaptively changed over atleast a portion of a cycle of the input voltage; and a control signalgenerator configured to control the current in the input circuit, thecontrol of the current based on a comparison of the sensed current andthe generated current limit.

In one embodiment, the current limit is determined from a sinusoidalcurrent limit profile over the at least the portion of the cycle of theinput voltage.

In one embodiment, the sinusoidal current limit profile is based on apeak value of the current limit, the peak value of the current limitbased on a peak value of the input voltage.

In one embodiment, the control signal generator comprises a comparatorconfigured to compare the current limit and the sensed current. Thecontrol signal generator is configured to output a control signal basedon the comparison.

In another embodiment, a method is provided comprising: sensing acurrent and input voltage from an input circuit; generating a currentlimit based on the input voltage of the input circuit, wherein thecurrent limit is adaptively changed over at least a portion of a cycleof the input voltage; and controlling the current in the input circuit,the control of the current based on a comparison of the sensed currentand the generated current limit.

In one embodiment, the current limit is determined from a sinusoidalcurrent limit profile over the at least the portion of the cycle of theinput voltage.

In one embodiment, the sinusoidal current limit profile is based on apeak value of the current limit, the peak value of the current limitbased on a peak value of the input voltage.

In one embodiment, the method further comprises: receiving the inputvoltage; determining a peak value of the input voltage and aninstantaneous angle of the input voltage; determining a peak value of asensed voltage, the sensed voltage being a margin above the peak valueof the input voltage; generating the current limit by applying the peakvalue of the sensed voltage to a sine of the instantaneous angle.

The following detailed description and accompanying drawings provide abetter understanding of the nature and advantages of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows two waveforms illustrating effects of the displacement anddistortion factors.

FIG. 2 shows a graph of input power vs. an input voltage V_(in rms) forconstant input current.

FIG. 3 depicts a simplified system for power factor correction accordingto one embodiment.

FIG. 4 depicts a more detailed example of the system according to oneembodiment.

FIG. 5A depicts a graph showing the sensed voltage V_(rsns) over thehalf cycle according to one embodiment.

FIG. 5B depicts a graph showing the current limit profile according toone embodiment.

FIG. 5C depicts a graph showing the instantaneous power obtained by thesinusoidal current limit and the constant limit according to oneembodiment.

FIG. 6 depicts a graph of the change in constant and sinusoidal currentlimits implementations according to one embodiment.

FIG. 7 depicts a graph of the relationship between a peak current limitand the input voltage V_(in rms) according to one embodiment.

FIG. 8 depicts an example of an adaptive current limiting according toone embodiment.

FIG. 9 depicts a more detailed example of the adaptive current limitingaccording to one embodiment.

FIG. 10 depicts a graph of two piecewise linear curves that may be usedto determine a peak of a sinusoidal current limit according to oneembodiment.

FIG. 11 depicts the variation of input power vs. the input voltageV_(in rms) according to one embodiment.

FIG. 12 depicts a simplified flowchart of a method for controlling thecurrent according to one embodiment.

DETAILED DESCRIPTION

Described herein are techniques for current limiting in power factorcorrection. In the following description, for purposes of explanation,numerous examples and specific details are set forth in order to providea thorough understanding of embodiments of the present invention.Particular embodiments as defined by the claims may include some or allof the features in these examples alone or in combination with otherfeatures described below, and may further include modifications andequivalents of the features and concepts described herein.

FIG. 3 depicts a simplified system 300 for power factor correctionaccording to one embodiment. System 300 includes an input circuit 302, aload 304, and a power factor corrector 306. A power supply, such as aswitch mode power supply, is also coupled to input circuit 302.

Power factor corrector 306 shapes an input current waveform of inputcircuit 302 such that it is sinusoidal and in phase with a voltagewaveform of input circuit 302. In one embodiment, power factor corrector306 receives a current I_(sns) that is sensed from input circuit 302.For example, the current I_(sns) may be sensed across a resistor ofinput circuit 302. Also, power factor corrector 306 receives an outputvoltage V_(fp) sensed across load 304. The sensed current I_(sns) andthe output voltage V_(fp) are used to shape the input current to bein-phase with an input voltage waveform and sinusoidal. For example, adistorted input current waveform is shaped to be sinusoidal like thevoltage waveform. Also, an input current that is out of phase with thevoltage waveform is shifted to be in phase with the voltage waveform. Aperson skilled in the art will appreciate how power factor correction isperformed based on the teachings and disclosure herein.

Particular embodiments are directed to limiting a maximum current ininput circuit 302 during the power factor correction. In one embodiment,power factor corrector 306 provides over-current protection (OCP).Limiting the maximum current in input circuit 302 protects system 300from being damaged. A current limit is used to limit the maximumcurrent. Particular embodiments adaptively change the current limit.Adaptively changing the current limit also limits variations in maximumoutput power limit, which further protects system 300. For example, thevariation in current is reduced, which reduces the variation in powerconsumed.

Power factor corrector 306 uses the sensed current I_(sns) to determinewhether to limit the current in input circuit 302. For example, powerfactor corrector 306 compares the sensed current I_(sns) to a generatedcurrent limit. If the current limit is exceeded, power factor corrector306 outputs a control signal that limits the current in input circuit302. For example, power factor corrector 306 stops a transistor fromturning on for a current pulse width modulation (PWM) cycle. The PWMcycle is the cycle of a signal that turns the transistor on and off.This ensures that the current through input circuit 302 does not exceeda maximum current defined by the current limit.

FIG. 4 depicts a more detailed example of system 300 according to oneembodiment. In one embodiment, input circuit 302 includes a flybackconverter. Although a flyback converter is described, power factorcorrector 306 may be used in other converter topologies. The flybackconverter may be used in notebook power supply adapters, such asswitched mode power supplies.

Input circuit 302 includes a diode bridge 404 and a capacitor 406. Load304 may be any load, such as a notebook computer. Load 304 includesinductors 408 a and 408 b, a capacitor 410, and resistors 412 a, 412 b,and 412 c. A metal-oxide-semiconductor field-effect transistor (MOSFET)402 and resistor 414 are also included to provide current limiting. Aperson of skill in the art will understand the operation of inputcircuit 302 and the flyback converter in accordance with the disclosureand teachings herein.

An input current I, flows through input circuit 302 and can be sensed atresistor 414. The sensed current is referred to as I_(sns). Also, theoutput voltage V_(fb) is sensed in between resistors 412 b and 412 c andreceived at power factor corrector 306 through an isolator 416. Theoutput voltage V_(fb) is used to determine amplitude of a referencecurrent that is compared with the input current. The comparison is usedto shape the input current to be sinusoidal with the input voltagewaveform in power factor correction.

MOSFET 402 is controlled by power factor corrector 306 to limit currentand also output power variation. For example, MOSFET 402 may be turnedoff if the current limit is reached in input circuit 302. Turning offMOSFET 402 stops current flowing through input circuit 302. AlthoughMOSFET 402 is described, it will be understood that any component may beused to stop current flow in input circuit 302.

Power factor corrector 306 determines the current limit that may beadaptively, for example, changed over a half cycle of the voltagewaveform. Although a half cycle is described, other portions of thecycle may be used. The adaptive current limit is compared to the sensedcurrent I_(sns) over the half cycle of the voltage waveform. In oneembodiment, if sensed current I_(sns) is greater than the current limitduring the half cycle, MOSFET 402 is turned off using a switching signal(SW). This limits the current in system 300. Although turning MOSFET 402off is described, it will be understood that other devices may be usedin limiting current.

In one embodiment, a current limit profile is used to dynamically changethe current limit. The current limit profile may be the range of valuesof the current limit of the half cycle. In one embodiment, the currentlimit profile is sinusoidal over the half cycle of the voltage waveform.The current limit profile may be determined by calculating a peak valueof the input current. A margin above the peak value of the input currentis set as the peak value of a sinusoidal current limit profile. Thecurrent limit profile is generated in-phase with the input voltage

The current limit profile reduces the average power limit and also thevariation in the power limit over the input voltage V_(in rms) range. Anexample of the current limit profile, power used, and power variationwill be described for a 36 W adaptor. FIG. 5A depicts a graph 500showing the voltage across resistor 414 over the half cycle according toone embodiment. A waveform 501 shows a half cycle of the voltage acrossresistor 414. The current limit is adaptively changed sinusoidally basedon the peak voltage calculated for each pulse of the voltage.

FIG. 5B shows a graph 502 the current limit profile according to oneembodiment. A first waveform 503 shows the peak current profile at 100%load. A second waveform 504 shows a sinusoidal current limit profileI_(pk) _(—) _(limit) with a 30% margin. The current limit profile I_(pk)_(—) _(limit) shows the current limit values in the half cycle. Althougha 30% margin is described, other margins may be used.

A waveform 506 shows a constant current limit that is conventionallyused. For example, the conventional constant current limit takes thepeak current from waveform 502 and adds a 30% margin onto the peakcurrent found at a point 508. The conventional current limit that isdetermined at point 508 is kept constant throughout the half cycle asshown in waveform 506.

The power obtained using the conventional constant current limit profileand the sinusoidal current limit profile is different. FIG. 5C depicts agraph 507 showing the instantaneous power obtained by the sinusoidalcurrent limit and the constant limit according to one embodiment. Awaveform 508 shows the instantaneous power (inst_pow_limit_sinu)obtained by the sinusoidal current limit profile shown in waveform 504of FIG. 5B. A waveform 510 shows the instantaneous power(inst_pow_limit_const) obtained by using the constant limit profileshown by waveform 506. In one example, the average power from theinstantaneous power profiles is 60 watts for the sinusoidal currentlimit profile and 90 watts for the constant profile limit. Average powermay be the area under each curve and the sinusoidal current limitprofile uses less average power.

Using the sinusoidal current limit profile also reduces the variation inpower limit. FIG. 6 depicts a graph of the change in constant andsinusoidal current limits implementations according to one embodiment. Awaveform 602 shows the variation in power using the sinusoidal currentlimit profile. As shown, a variation of 20 watts of power occurs overthe range of the input voltage V_(in rms). A waveform 604 shows thepower used with the conventional constant current limit profile. Asshown, the average power varies over 40 watts for the range of the inputvoltage V_(in rms) for the conventional constant current limit profile.

Particular embodiments use the peak current for a given rms inputvoltage V_(in rms) to determine the current limit profile. In oneembodiment, the current limit profile is 130% of a calculated peakcurrent. The peak current varies with respect to the input voltageV_(in rms). For example, if the input voltage V_(in rms) is known, thenpeak current I_(pk) can be determined. Then, the current limit I_(pk)_(—) _(limit) is determined A relationship between V_(in rms) and thecurrent limit profile can thus be determined from the peak currentI_(pk). FIG. 7 shows a graph 700 of the relationship between the currentlimit and the input voltage V_(in rms) according to one embodiment. Awaveform 702 shows the values of a peak value of the current limitprofile within the range of the input voltage V_(in rms). If the valueof V_(in rms) is known, then the peak value of the current limit profilecan be determined.

The above relationship between the peak value of the current limitprofile and the input voltage V_(in rms) may be used by power factorcorrector 306 to limit the current in system 300. FIG. 8 shows moredetailed example of adaptive current limiting in power factor corrector306 according to one embodiment. A reference generator 802 receives avalue of the input voltage V_(in rms). This is the input voltage atinput circuit 302. Reference generator 802 can then determine a peakvalue of the current limit based on the input voltage. A current limitprofile is then used to determine a current limit to be used in acomparison with the sensed current I_(sns). This process will bedescribed in more detail below.

A control signal generator 804 receives the current sensed I_(sns)across resistor 404. Control signal generator 804 may compare the sensedcurrent I_(sns) and the current limit. Based on the comparison, thecontrol signal may turn off MOSFET 402 to limit the current. Forexample, if the sensed current I_(sns) exceeds the current limit, thenthe control signal may turn off MOSFET 402. This limits the inputcurrent. In one embodiment, the input current may be turned off for theremainder of the pulse width modulation (PWM) cycle. The PWM cycle isthe signal that power factor corrector 306 outputs to MOSFET 402 toswitch MOSFET 402 on and off. In this case, no more power transferoccurs across load 304. The comparison may be determined at every pulseof the sensed current.

The generation of the current limit sent to control signal generator 804will now be described in more detail. FIG. 9 depicts a more detailedembodiment of adaptive current limiting in power factor corrector 306according to one embodiment. V_(in) computational logic 902 receives theinput voltage V_(in rms). A one bit signal (N, M) may be sent to apredictive input sine block 904 indicating a digital representation ofthe input voltage V_(in). This moves from processing in the analogdomain to the digital domain. Predictive input sine block 904 determinesa peak value of input voltage V_(in) and also an instantaneous angle θof the input voltage V_(in). The instantaneous angle θ and peak valueV_(pk) of the pulse may be used to determine the current limit.

The peak input voltage V_(pk) is sent to a voltage V_(rsns) generator906. V_(rsns) may be the voltage sensed across resistor 414. The peakinput voltage V_(pk) is used to determine the peak value of voltageV_(rsns) (peak value). A chip may process voltage values. That is, it isthe voltage value that corresponds to the peak value of the currentlimit profile discussed above.

The value of the voltage V_(rsns) (peak value) may be determined indifferent ways. In one embodiment, a look-up table may be used. Forexample, a look-up table includes the values of the peak input voltageV_(pk) and corresponding voltages for the voltage V_(rsns) (peak value).In another embodiment, an equation may be used. For example, FIG. 10shows a graph 1000 of two piecewise linear curves that may be used todetermine the peak of a sinusoidal current limit according to oneembodiment. Two piecewise linear curves 1002 a and 1002 b are usedinstead of a continuous non-linear curve because the non-linear curvemay yield values that are too general. Although two piecewise linearcurves are shown, it will be understood that other equations may be usedto determine the voltage V_(rsns) (peak value).

The two piecewise linear curves may be used to approximate therelationship of the voltage V_(rsns) to the input voltage V_(in rms).For example, the value of the voltage V_(rsns) with respect to the inputvoltage V_(in rms) are approximated using piecewise linear curves 1002 aand 1002 b. Using the equations for piecewise linear curves 1002 a and1002 b, the value of the peak input voltage V_(pk) may be used todetermine the value of the voltage V_(rsns) (peak value).

Referring back to FIG. 9, a current limit generator 908 receives theinstantaneous angle θ from predictive input sine logic 904 and the valueof the voltage V_(rsns) (peak value) from V_(rsns) generator 906.Current limit generator 908 applies the value of the voltage V_(rsns)(peak value) to a sinusoidal profile to determine the current limit. Forexample, the angle is used to apply a sinusoidal profile to the value ofthe voltage V_(rsns) (peak value) to determine a current limit. Theequation V_(rsns) (peak value)*sin(θ) may be used to determine the valueof the current limit. For example, once the voltage profile from theequation is determined, it can be converted to a current profile basedon a value of resistor 414, which is the current limit profile. As thevoltage V_(rsns) (peak value) and the instantaneous angle θ vary, thevalue of the current limit varies.

Current limit generator 908 determines the current limit in the digitaldomain. A digital-to-analog converter (DAC) 910 receives the currentlimit and converts it to an analog signal for a comparison.

A comparator 912 receives the current limit and the sensed currentI_(sns). A comparison is performed to determine if the sensed currentI_(sns) exceeds the current limit.

Comparator 912 outputs a control signal based on the comparison. Forexample, the control signal may turn off MOSFET 4-402 if the currentlimit is exceeded by the sensed current I_(sns). In one example, if thecurrent limit is exceeded in the PWM cycle, MOSFET 4-402 is turned offimmediately for the rest of the PWM cycle thereby limiting further powertransfer and protecting system 3-300.

Using the example of adaptive current limiting in power factor corrector3-306 shown in FIG. 9 and the piecewise linear model of FIG. 10, thevariation of the input power is reduced from 20 watts to 6 watts. FIG.11 shows the variation of input power vs. the input voltage V_(in rms)according to one embodiment. A first waveform 1102 a shows the variationin power when equation 10-1002 a is used to determine the voltageV_(rsns) (peak value). A waveform 1102 b shows the input power when theequation corresponding to equation 10-1002 b is used. As shown, avariation of 6 watts occurs in the input power using piecewise linearequations 10-1002 a and 10-1002 b. The power variation is different inFIG. 11 as compared to FIG. 6 because the peak of the sinusoidal currentlimit is changed based on equations 10-1002 a and 10-1002 b. In FIG. 6,the current limit is constant for all V_(in rms).

A waveform 1104 shows a theoretical power limit. The theoretical powerlimit is 130% of the input power. This is the theoretical power limitoccurs when the current limit is sinusoidal and the peak value of thesinusoidal current limit is changed according to the input voltageV_(in rms) accurately.

FIG. 12 depicts a simplified flowchart 1200 of a method for controllingthe current according to one embodiment. At 1202, the input voltageV_(in rms) is received. At 1204, a peak input voltage V_(pk) isdetermined.

At 1206, the value of the voltage V_(sns) (peak value) is determined. At1208, a current limit is generated by applying a sinusoidal profile tothe value of the voltage V_(sns) (peak value). In one embodiment, thecurrent limit may be converted from a digital to analog value.

At 1210, the sensed current I_(sns) is received. At 1212, a comparisonof the sensed current I_(sns) and the current limit may then beperformed. At 1214, the control signal is generated based on thecomparison of sensed current I_(sns) and the generated current limit.The above method is performed over the half cycle of the input voltage.

As used in the description herein and throughout the claims that follow,“a”, “an”, and “the” includes plural references unless the contextclearly dictates otherwise. Also, as used in the description herein andthroughout the claims that follow, the meaning of “in” includes “in” and“on” unless the context clearly dictates otherwise.

The above description illustrates various embodiments of the presentinvention along with examples of how aspects of the present inventionmay be implemented. The above examples and embodiments should not bedeemed to be the only embodiments, and are presented to illustrate theflexibility and advantages of the present invention as defined by thefollowing claims. Based on the above disclosure and the followingclaims, other arrangements, embodiments, implementations and equivalentsmay be employed without departing from the scope of the invention asdefined by the claims.

1. An apparatus comprising: an input configured to sense a current andinput voltage from an input circuit; a reference generator configured togenerate a current limit based on the input voltage, wherein the currentlimit is adaptively changed over at least a portion of a cycle of theinput voltage; and a control signal generator configured to control thecurrent in the input circuit, the control of the current based on acomparison of the sensed current and the generated current limit.
 2. Theapparatus of claim 1, wherein the current limit is determined from asinusoidal current limit profile over the at least the portion of thecycle of the input voltage.
 3. The apparatus of claim 2, wherein thesinusoidal current limit profile is based on a peak value of the currentlimit, the peak value of the current limit based on a peak value of theinput voltage.
 4. The apparatus of claim 3, wherein the referencegenerator determines the current limit based on an angle of the inputvoltage applied to the sinusoidal current limit profile.
 5. Theapparatus of claim 3, wherein a peak value of the sinusoidal currentlimit profile is a margin above the peak value of the current.
 6. Theapparatus of claim 5, further comprising a look up table used by thereference generator to determine the peak value of the sinusoidalcurrent limit profile.
 7. The apparatus of claim 1, wherein the cyclecomprises a half cycle.
 8. The apparatus of claim 1, wherein the controlsignal generator comprises a comparator configured to compare thecurrent limit and the sensed current, wherein the control signalgenerator is configured to output a control signal based on thecomparison.
 9. The apparatus of claim 1, wherein the control signalturns off a component in the input circuit to limit current through theinput circuit.
 10. The apparatus of claim 9, wherein the componentcomprises a MOSFET.
 11. The apparatus of claim 9, wherein the componentis turned off during a pulse width modulation cycle of a pulse widthmodulation signal being input to component.
 12. A method comprising:sensing a current and input voltage from an input circuit; generating acurrent limit based on the input voltage of the input circuit, whereinthe current limit is adaptively changed over at least a portion of acycle of the input voltage; and controlling the current in the inputcircuit, the control of the current based on a comparison of the sensedcurrent and the generated current limit.
 13. The method of claim 12,wherein the current limit is determined from a sinusoidal current limitprofile over the at least the portion of the cycle of the input voltage.14. The method of claim 13 wherein the sinusoidal current limit profileis based on a peak value of the current limit, the peak value of thecurrent limit based on a peak value of the input voltage.
 15. The methodof claim 14, wherein a peak value of the sinusoidal current limitprofile is a margin above the peak value of the current.
 16. The methodof claim 14, further comprising determining the peak value of thesinusoidal current limit profile based on a look-up table or anequation.
 17. The method of claim 13, wherein the current limit is basedon an angle of the input voltage applied to the sinusoidal current limitprofile.
 18. The method of claim 12, further comprising: comparing thecurrent limit and the sensed current; and outputting a control signalbased on the comparison.
 19. The method of claim 12, further comprising:receiving the input voltage; determining a peak value of the inputvoltage and an instantaneous angle of the input voltage; determining apeak value of a sensed voltage, the sensed voltage being a margin abovethe peak value of the input voltage; generating the current limit byapplying the peak value of the sensed voltage to a sine of theinstantaneous angle.
 20. The method of claim 12, wherein the cyclecomprises a half cycle.